FAA issued the proposal AD in August, citing investigations into numerous reports of cylinder head-to-barrel separations and cracked and leaking aluminum cylinder heads on the affected assemblies. The agency has worked with NTSB and industry for years on problems with cracked cylinder heads, and NTSB last year issued a recommendation for inspection and replacement of certain ECI assemblies.

But NTSB, in comments submitted this month, stated FAA’s proposed action “would affect many more cylinder assemblies than the NTSB included in our recommendation letter. Because we are not aware of information to support the expanded scope and decrease in compliance time contained in the proposed AD, we support FAA action more consistent with NTSB Safety Recommendation A-12-7.”

NTSB’s comments were welcomed by industry, which have argued the AD, which is estimated to cost $82.6 million, would impose an enormous burden with little evidence to support the scope of the requirements.

“It’s very unusual for the NTSB to weigh in on a proposed airworthiness directive in this manner after it issued a recommendation,” says Rob Hackman, Aircraft Owners and Pilots Association vice president for regulatory affairs. “Then again, it is rare for the FAA to propose going so far beyond an NTSB recommendation.”

Hackman adds that “We’re worried that it could dramatically raise the cost to aircraft owners and even create more safety problems than it solves by forcing the replacement of thousands of cylinders in the field.” AOPA and seven other aviation organizations had asked FAA to withdraw the proposal until the agency could provide evidence supporting the scope of the AD.

FAA acknowledged the potential impact when it released the AD and said it conducted a detailed review of all information it had received. But the agency added that after a risk assessment, “we concluded that proceeding with this proposed AD to correct the unsafe condition was appropriate.”

NTSB noted that it has not received concerns in investigated cases of problems with certain serial numbers covered by the proposed AD, while others included in the AD are already addressed through a more stringent AD that was previously released. And for most covered assemblies with serial numbers 36210-61117, NTSB says it “is not aware of any data that support including [those] cylinder assemblies in the proposed AD for inspection or removal.”

NTSB adds, however, that if FAA does have additional data to warrant expanding the scope of the proposed AD, the agency should furnish it for review.

FAA originally set a comment deadline of Oct. 11, but pushed it back to Dec. 11 to provide more time to review additional information the agency planned to add to the docket. So far more than 500 comments have been filed.